1. Field of the Invention
The invention relates to a sheet feeding clutch mechanism for use in an automatic sheet feeding unit of a printer, and more particularly, to a sheet feeding clutch mechanism which rotates a sheet feed shaft only in one direction, and ensures the positioning of a drive disk with respect to a clutch disk.
2. Description of the Related Art
Conventionally, sheet feeding clutch mechanisms of this type have been used in printers incorporated in computer systems, office-automation systems, and the like, as is described in Japanese Patent Disclosure No. 1780/82.
In an automatic sheet feeding unit provided with such a conventional clutch mechanism, a drive unit in a printer unit provides both a driving force for feeding unprinted paper sheets from a first magazine to a platen of the printer unit, and a driving force for feeding printed sheets from the platen into a second magazine. Also, a control device in the printer unit sends control commands for feeding these sheets.
The sheets are fed from the first magazine to the platen of the printer unit by feed rollers of the sheet feeding unit. After being caught by the platen, the paper sheets are line-fed or fed to the second magazine by the forward rotation of the platen. During the forward rotation of the platen for feeding the sheets, therefore, the feed rollers of the sheet feeding unit must be disengaged from the drive system.
As is disclosed in Japanese Patent Disclosure No. 8054/85, a one-way clutch is generally used to engage a sheet feed shaft with a drive system and to disengage the shaft from the system. In the conventional clutch of this type, a drive disk is rotatably mounted on the sheet feed shaft, which is rotatably supported by the sheet feeding unit. The drive disk includes a gear which meshes with a gear train operatively connected to the platen. An endless guide groove is formed in the outer peripheral surface of a clutch disk, which is rotatably mounted on the sheet feed shaft and is coaxially arranged adjacent to the drive disk. Formed in the guide groove are a stop used for free forward rotation of the clutch disk, and a slit at a predetermined angular distance from the stop. A ratchet pawl is rockably mounted on the drive disk by means of a pin, and is urged by a spring so that its tip is pressed against the guide groove. Guide surfaces are formed on the respective end of a range covering the slit. The guide surface on the upstream end thereof, when viewed in relation to the direction of forward rotation of the clutch disk, guides the ratchet pawl to the other guide way than the slit when the guide surface on the other end of the slit range leads the ratchet pawl to the slit when the drive disk rotates reversely. Through the slit, the pawl engages a drive gear fixed to the feed shaft in the clutch disk. After this engagement, the forward rotation of the drive disk transmits the rotation of the drive disk to the drive gear, enabling the clutch mechanism to be in an "on" position.
In starting the printer with the conventional sheet feeding clutch, after filling the first magazine with paper sheets, the platen shaft is first manually rotated forwardly or in the sheet feeding direction through a desired angle (for example, 180.degree.), whereupon the drive disk rotates in the same direction. After the tip of the ratchet pawl has engaged the stop, the clutch disk rotates in the same direction, with the pawl abutting against the stop. Thus, the positional relation between the clutch disk and the drive disk is settled.
A switch for automatic operation of the printer unit is turned on. Then, platen rotates in accordance with a program stored in a control device. The drive disk first rotates reversely through a predetermined angle, so that the ratchet pawl disengages from the stop and slides along the guide groove until it drops into the slit, thereby engaging the drive gear.
In response to a subsequent forward-rotation command from the program, both the platen and the drive disk rotate forwardly. At the same time, the drive disk and the drive gear which are engaged with the former by way of the ratchet pawl, rotate forwardly as a unit. As a result, the sheet feed shaft rotates, so that the topmost one of the paper sheets in the first magazine is delivered to the platen. When the sheet reaches the platen, the feed shaft ceases rotating. Thereafter, the drive disk is rotated reversely by the platen in response to a reverse-rotation command, whereby the clutch is in an "off" position.
Then, the program proceeds to a normal printing process. In this state, the platen, along with the drive disk, is rotated forwardly. Since the clutch is an "off" position, however, this rotation is not transmitted to the sheet feed shaft.
When the relative positional relation between the drive disk and the clutch disk is established before the starting of the printer, in the sheet feeding clutch described above, it is possible that the ratchet pawl may drop into the slit and engage the drive gear, thus making the clutch in the "on" condition and feeding sheets as the drive disk rotates forwardly. In such a case, the clutch cannot be initialized, and even if the printer unit is not in a printing mode, the sheets are erroneously fed.
Disclosed in German Patent Publication No. 2910849C2 is a sheet feeding apparatus mounted on a printer, wherein a paper sheet is fed backwardly by a distance corresponding to a desired number of lines on the sheet, during a printing operation, in order to print, for example, an exponent on the sheet. This apparatus, however, has a problem in that the number of lines to be returned can be changed only when a pin or sector is replaced.